In the field of machine tools and the like, various schemes for detecting a displacement amount of a movable body have been examined and put in a practical use. For example, Japanese Patent No. 3967963 (Patent Literature 1) discloses a device including two resolvers that are different in absolute detection period (FIG. 1) to detect absolute displacement by using displacement detection signals outputted from the resolvers. The detecting device converts two periodic signals into data that is for acquiring a periodic signal of a least common multiple of these signals, and detects absolute displacement by using digitized stroke values prestored in a ROM.
Further, Japanese Patent No. 3665732 (Patent Literature 2) discloses a device that uses a gear mechanism including a main shaft driving gear and three driven gears engaged therewith to detect an absolute position in multi turns of the main revolving shaft (FIG. 2). The detecting device obtains the number of rotations of each driven gear based on mechanical angles detected by a resolver joined with the three driven gears, and compares the obtained number of rotations with a relation (FIG. 9) between the number of rotations of each driven gear and the number of rotations of the main rotating shaft prestored in a ROM to detect an absolute position of the main revolving shaft.